This invention relates generally to rotational sensors and particularly to ring laser gyroscopes with improved reliability and more particularly to an apparatus and method for decreasing or preventing the degradation of the cathode vacuum seal caused by electromigration of mobile positive ions under the influence of the anode to cathode electric field during laser operation.
A ring laser gyroscope is an instrument which employs the Sagnac effect to measure rotation rate. The Sagnac effect results in a phase difference between two counterpropagating ring laser beams when the ring laser mirror assembly experiences solid body rotation. The Sagnac effect phase difference is proportional to the applied rotation rate and can be measured interferometrically to great precision, providing a sensitive measure of rotation. Electrically pumped helium-neon lasers arc typically used for gyroscope applications. The gyroscopic ring laser resonant cavity is defined by three or more mirrors which direct light in a closed path. It is desirable for stable single mode laser operation that the total optical path length of the closed path resonant cavity be held constant to within a small fraction of the laser wavelength. Since the instrument will generally be utilized in a variable temperature environment, the frame of the ring laser is preferably made from a low thermal expansion material. The frame material must also withstand the high voltage needed to energize the laser plasma and must resist also withstand the high voltage needed to energize the laser plasma and must resist the diffusion of laser gas out of the sealed laser assembly. Lithium-aluminum silicate based glass-ceramic materials, such as Zerodur, possess all of the necessary properties and are the preferred materials of construction for ring laser gyroscope frames. One of the few disadvantages of lithium aluminum silicate glass ceramics is that they tend to have ionic conductivity values larger than many other nominally dielectric materials. This ionic conduction property can cause a reduction in gyro life and reliability at operating temperatures of 40 degrees Celsius and above.
In a typical method of construction, a ring laser gyroscope is built from a block or frame of lithium-aluminum silicate glass-ceramic which has been carefully machined and finished to provide three or more hollow cylindrical laser bores which define the plasma chamber and form the laser resonator closed path, three or more corner surfaces to receive the laser mirrors, and two or more additional openings to the surface of the frame. These additional openings are interconnected with the plasma chamber and receive the anode and cathode electrodes needed to energize the laser plasma in the bores. In this method of construction the anode and cathode electrodes are attached to the outer surface of the frame over the appropriate openings using a cold formed or fused indium vacuum seal. The anode and cathode electrodes are typically made from metals with thermal expansion coefficients greater than that of the lithium-aluminum silicate glass-ceramic frame. Indium metal is generally chosen for the vacuum seal because of it's unique ability to adhere to the dissimilar ceramics and metal joined parts. Indium metal also deforms plastically in response to differential thermal expansion motions of the joined parts without losing vacuum tightness and without applying large traction forces to the laser frame. It is important for the reliability of the instrument that the indium vacuum seal be completely impervious to the passage of laser gases out of the plasma chamber and also impervious to the passage of atmospheric gases into the chamber. The indium seal must also be mechanically sound and free of voids in order to withstand repeated thermally induced deformations during the life of the instrument.
The inventor has observed the indium seals under the cathode electrodes chemically and mechanically degraded after long periods of laser operation at elevated temperatures. Evidence of this degradation can be observed visually as a change in the appearance of the interface between the indium metal and the glass-ceramic frame. It can also be measured directly as a reduction in the force required to detach the cathode from the frame. Also the propensity of the cathode seal to leak as a result of thermal cycling is increased as a result of long exposure to a combination of high temperature and high voltage.
When the indium portions of such degraded or failed seals arc examined by the methods of surface science and chemical analysis, significant amounts of lithium are found in the indium in the region adjacent to the laser frame. It is evident from the patent and technical literature sources cited that this lithium has been transported by ionic conduction as Li.sup.+ through the volume or along the surface of the laser frame and deposited electrolytically on the cathode indium seal. There is reason to believe that electromigration of other positive ions including H.sup.+ or H.sub.3 O.sup.+ (hydronium) may also contribute to cathode seal degradation. While the exact mechanisms of seal degradation are not known with certainty, the general weakening of the seal caused by the electromigration of positive ions causes a reduction in the life and reliability of ring laser gyroscopes operated for long periods at elevated temperatures.
Degradation of ring laser gyroscope cathode seals by electromigration of positive ions is a phenomenon known in the art. For example, Karlbeinz vonBieren in U.S. Pat. No. 5,098,189, entitled "ION-SUPPRESSED RING LASER GYRO FRAMES", describes a similar problem. vonBieren teaches the use of specially formed gaps in the laser frame and/or auxiliary electrodes held in proximity to but not contacting the frame for the purpose of modifying the electric field in such a way as to suppress the flow of ions. Similarly, Canfield et. al., in U.S. Pat. No. 5,432,604, teach the use of specially placed insulating barriers to suppress ion transport in ring laser gyroscope frames.
While the air gap and insulating barrier methods taught in these two patents may be somewhat effective in their intended purposes, they have the disadvantage of increasing the number of precisely machined parts and complicating the gyro assembly process. Both of these disadvantages add cost and may act to reduce the reliability of the final instrument produced. The non-contacting auxiliary negative electrodes, taught by vonBieren, will probably not be effective since the desired electric field distortion thereby produced will quickly be canceled by an opposing electric field produced by positive ions attracted to the frame surface near the auxiliary electrodes. This field cancellation phenomenon, known as electrode polarization, is observed in ionically conducting systems in which one or both electrodes lack the ability to supply new ions to the ionically conducting medium or to absorb and neutralize the arriving transported ions. Thus, the non-contacting, negatively charged electrodes, taught by vonBieren, lack the ability to absorb and neutralize the attracted positive ions which will therefore accumulate producing a positive charge offsetting the negative charge on the electrode